1. Field of the Invention
This invention relates to new polydiorganosiloxane compositions that are curable with radiation; to a method of preparing a coated substrate using said compositions and to the coated substrate produced thereby.
In particular, this invention relates to certain mercaptoalkyl- and vinyl-containing polydiorganosiloxane compositions which, when applied to a substrate such as paper and exposed to energetic radiation such as ultraviolet light, will cure rapidly to produce a coating, adhered to the substrate, that provides premium release of aggressive acrylic adhesives.
2. Description of the Prior Art
Polyorganosiloxanes have been used for a long time to coat solid substrates, such as paper, to improve the release of adhesive materials therefrom. As the adhesive properties and holding power of adhesive materials have improved, the need for improved release coatings has increased. Improved release coatings have also been required by other factors such as increasing environmental concerns, a desire for faster coating and curing processes, and a need to lower process energy costs.
Solventless, radiation-curable coatings have thus been sought that would provide premium release of aggresive acrylic adhesives. A coating is considered to provide premium release if a force of not more than 100 grams per inch (38.61 newtons per meter), as measured by the method hereinafter described, is needed to remove the adhesive from the coating. An aggressive adhesive is a material that requires a force of at least approximately 460 newtons/meter (N/m) to remove the adhesive from a stainless steel surface using said method of measuring.
Mercaptoalkyl- and vinyl-substituted silanes, siloxanes and polysiloxane compositions are not new; but only recently has the reaction of silicon-bonded mercaptoalkyl radicals with silicon-bonded vinyl radicals been used to convert a fluid polyorganosiloxane composition to a solid material having useful properties such as hardness and adhesive release.
Low-temperature, fast-curing electrical and coating resins consisting essentially of certain mercaptoalkyl-containing siloxane resins, a methylvinylpolysiloxane compatible with the resin and certain gelation inhibitors are disclosed and claimed by Michael, et al., in U.S. Pat. No. 3,873,499. The compositions of Michael, et al., are curable with ultraviolet light, electron radiation or free-radical generators. Without the gelation inhibitor the mixture of resin and polymethylvinylsiloxane will spontaneously cure at room temperature in a relatively short period of time.
Viventi, U.S. Pat. No. 3,816,282, discloses and claims an organopolysiloxane composition, curable at room temperature in the presence of electromagnetic and particulate radiation, comprising a high-molecularweight organopolysiloxane wherein a small percentage, for example from 0.1 to 2 percent, of the siliconbonded organic radicals are vinyl; an organopolysiloxane fluid having mercaptoalkyl substituents and from 0.025 to 1 percent by weight of certain free-radical photosensitizers. The compositions of Viventi can be cured in a period of time as short as 5 to 10 minutes or as long as 2 to 4 hours. Even 5 minutes is an undesirably long cure time for many commercial applications, especially in the paper coatings industry. A curable composition with a cure time of less than 60 seconds, and preferably less than 5 seconds, is desired.
Gant, in U.S. application Ser. No. 401,791 filed Sept. 28, 1973, entitled "UV Curable Composition" and assigned to the assignee of this invention, discloses and claims a composition, curable on exposure to ultraviolet light, consisting essentially of a siloxane having silicon-bonded hydrogen atoms and/or a mercapto-functional-containing siloxane; a vinyl-containing siloxane and a photosensitizer. In the compositions of Gant the mercapto-functional-containing siloxane can be any siloxane consisting essentially of from 0.1 to 100 mole percent of units of the formula (HSR").sub.y (R'").sub.x SiO.sub.(4-x-y)/2, any other siloxane units having the formula R"".sub.z SiO.sub.(4-z)/2, wherein R" is an alkylene radical, R'" and R"" are monovalent hydrocarbon radicals and/or monovalent halogenated hydrocarbon radicals, x, is 0-2, y is 1 or 2, z is 0-3, and x + y is 1-3. The vinyl-containing siloxane in the compositions of Gant consists essentially of from 0.1 to 100 mole percent of units of the formula (CH.sub.2 =CH)R.sub.n SiO.sub.(3-n)/2, any non-vinyl containing units having the formula R'.sub.m SiO.sub.(4-m)/2 wherein R and R' are monovalent hydrocarbon radicals and/or monovalent halogenated hydrocarbon radicals, n is 0-2 and m is 0-3. For every mercaptoalkyl radical there is from 0.01 to 100 vinyl radicals in the composition of Gant.
Gant, in U.S. application Ser. No. 401,792 filed Sept. 28, 1973, entitled "Improving Release Characteristics of Substrates" and assigned to the assignee of this invention, discloses and claims a method of treating a solid substrate to improve release of adhesive materials therefrom comprising applying the compositions of Gant that are disclosed in U.S. application Ser. No. 401,791 to said substrate and curing the applied composition by exposing it to ultraviolet light.
Certain compositions within these broad boundaries have been shown by Gant to be useful as a U.V.-curable paper coating for releasing adhesive materials. However, these compositons do not provide the premium release that is needed for aggressive acrylic adhesives, especially acrylic adhesives of the curing types.